1. Field of the Invention
The present invention relates to a method of cleaning a process tube, particularly capable of efficiently removing low density films to consist chiefly of silicon or silicon system compound from an inner wall of the process tube and a surface of other members.
2. Description of the Related Art
Usually in the course of manufacturing semiconductor devices, normal pressure or pressure-reduced CVD apparatuses are used to form on each of semiconductor wafers one of films of the poly-crystal-silicon system (Poly-Si), silicon nitride (SiN) system, silicon oxide (SiO.sub.2) system and tungsten silicide (WSix) system.
At the process of forming the film of the SiO.sub.2 system compound on the wafers, for example, the wafer boat is loaded into the process tube of the pressure-reduced CVD apparatus and heated there under reduced pressure. The mixture of SiH.sub.4 and O.sub.2 gases is then introduced into the process tube to form the film of SiO.sub.2 on the silicon wafers. Upon loading and unloading the wafer boat into and out of the process tube in this case, the process tube is heated to a temperature substantially equal to the film-forming process temperature.
When the above-described film-forming process is carried out, SiO.sub.2 adheres to the inner wall of the process tube and to other quartz-made tools. When this layer of SiO.sub.2 develops on the inner wall of the process tube as the film-forming process is repeated, it peels off the inner wall, scattering its particles in the process tube.
Particularly upon loading and unloading the wafer boat into and out of the process tube, more particles scatter in the process tube at and near the opening (inlet) thereof. The scattering particles come to adhere to the wafers, thereby lowering the productivity of the wafers. This is because temperature in the process tube becomes lower at and near the opening of the tube than that at the uniformly-heated zone in the process tube so that amorphous products reacted of low density can be created there. In other words, the reason resides in that the force with which these amorphous products reacted of low density adhere to the inner wall of the tube is quite smaller. They therefore peel off and drop from the inner wall even when they are slightly contacted with other things and slightly vibrated together with the tube.
Japanese Patent Disclosures Sho 64-17857 and Hei 1-92385 disclose methods of plasma cleaning the reacted products stuck to the inner wall of the process tube. According to these methods, the reacted products stuck are removed by dry etching.
In the case this plasma dry etching is used, however, coils and electrodes must be located around the process tube to generate plasma. These coils and electrodes disturb the forming of the uniformly-heated zone in the process tube at the time of the film-forming process. In addition, the process tube must be detached from the apparatus itself upon cleaning the process tube. This process tube is quite heavy and it takes a long time to detach the tube from the apparatus and to clean it. Usually, the apparatus must be stopped for 24 hours or more upon cleaning the inner wall of the tube. This causes the operating efficiency of the apparatus to be lowered to a great extent. Particularly when the size of wafers to be processed is expected to change from 6 inches to 8 inches in the near future, the process tube will be larger-sized, thereby making it more difficult to detach the process tube from the apparatus itself.
ClF.sub.3 gas is high in reaction. When it is used as cleaning gas, therefore, it can be expected that the rate of etching the reacted products becomes quite high at the time of the cleaning process. However, those reacted products stuck to that portion of the inner wall of the process tube which is in the uniformly-heated zone (where the wafers are film-forming-processed) are quite high in density. Even when ClF.sub.3 gas is used, therefore, the reacted products cannot be fully removed for a short time unless they are etched under high temperature. Further, the process tube has a thickness up to about 5 mm. When the etching rate is set too high, therefore, the process tube itself may be etched, and the thickness of the process tube decreases. As a result, the process tube is liable to bread off.